1. Field of the Invention
The present invention relates to a lightning arrester comprising resistance elements made of zinc oxide as a main component, having a nonlinear voltage-current characteristic and being disposed in an insulating cylinder, and more particularly to a lightning arrester having a pressure relief structure in which an excessive gas pressure that is produced in the above-mentioned insulating cylinder upon occurrence of an arc accident is discharged to the outside from the inside of the insulating cylinder.
2. Description of the Prior Art
A conventional pressure relief structure in a lightning arrester in which resistance elements 1 having a nonlinear voltage-current characteristic (which will be hereinbelow denoted "nonlinear elements") are disposed in an insulating cylinder is shown in FIG. 1 (refer to Japanese Patent Application Laid-open No. 61-151913). External threads are formed on outer peripheral surfaces of both end parts of an insulating cylinder 2 made of reinforced plastic material whose mechanical strength is enhanced by use of glass fibers and disposed therein with the nonlinear elements, and a plurality of pressure relief holes 6 are formed in the insulating cylinder 2 on each of circumferences at a plurality of positions in a longitudinal direction. Cap-like metal flanges 3, 13 having on their inner peripheral surfaces internal threads adapted to be engaged with the above-mentioned external threads are screwed onto both end parts of the insulating cylinder while they press a Belleville spring 4, and the nonlinear elements are held in the insulating cylinder and exert on one another a predetermined contact pressure. An insulating pipe 5 prpovided thereon with a plurality of ring-like fins 5a, for sealing the pressure relief holes 6 in the insulating cylinder 2 in a gas-tight manner so as to prevent moisture from entering into the cylinder, and for ensuring external insulation for the insulating cylinder 2, is fill-molded on the outer peripheral surface of the insulating cylinder, by using casting resin such as elastic organic insulating materials, for example, silicon rubber or the like. The above-mentioned fill-molding is made such that the casting resin is filled in the inside space of the insulating cylinder 2, and the resin covers the nonlinear elements in a gas-tight manner, and therefore, the nonlinear elements are shielded from moisture which is liable to enter arrester from outside along the inner peripheral surface of the metal flanges 3, 13 thereby it is possible to prevent the nonlinear elements from being deteriorated.
In the thus formed lightening arrester, when excessive surge current exceeding a designed value runs through the nonlinear elements, the nonlinear elements are subjected to a through-breakage or a surface creepage type spark short-circuit, and therefore, the temperature of the casting resin covering the nonlinear elements is raised by arc-heat generated at that time so that the casting resin is softened, or melted. This softening and melting of the reaches the pressure relief holes 6 in the insulating cylinder 2 in a short time thereby causing the softening or melting of the thick wall parts of the insulating pipe 5, corresponding to the pressure relief holes 6. Thereby, gas in the insulating cylinder 2 is discharged to outside through the pressure relief holes 6, and therefore, it is possible to prevent the lightning arrester from exploding.
The thus arranged lightning arrester eliminates the necessity of the provision of a fracture plate which normally closes one end of the insulating cylinder and is fragmented by the internal pressure in the insulating cylinder upon an arc-accident so as to discharge gas to the outside. A discharge section stores the fracture plate and forms a passage for the discharged gas in a conventional lightening arrester. Further, a pressure discharge function may be added as the function of a entire arrangement of the lightning arrester allowing the lightning arrester to be miniaturized in its entirety. However, a lightning arrester having the arrangement shown in FIG. 1 has the following problems:
(1) Even though several pressure relief holes are formed in the insulating cylinder, the pressure relief holes are not practically opened so as to communicate the inside of the cylinder with the outside uniformly at all positions thereof. For example, in the case of an accident of surface creepage spark short-circuit of the nonlinear elements as an arc-accident in the insulating cylinder, the casting resin fills in the pressure relief holes at positions which are located near the spark short-circuit, is readily softened and melted, but the casing resin in the pressure relief holes at positions which are out of a range of the spark short-circuit are not readily softened and melted. Accordingly, the discharge of internal pressure is insufficient as a whole, and since strong gas discharge is continued through the pressure relief holes alone in which the resin is softened and melted, a large insulating space is required around the lightening arrester.
(2) Since behavior of internal arcs is complicated, the pressure is locally not uniform, and the cast resin which fills the pressure relief holes in the part where the pressure is high is pushed out at first so that an excessive pressure condition is continued while an insufficient pressure discharge condition remains. Therefore it is difficult to obtain several opened pressure relief holes.